Signal conditioning in voice communication, particularly telephone communication, primarily addresses the problems of weak incoming electrical voice signals and large amplitude electrical signals which produce acoustic shock to the listener's eardrum. To alleviate the problem of weak incoming electrical voice signals, additional receiver gain is beneficial. To obviate acoustic shock due to incoming electrical signals of excessive amplitude, compression of the incoming signal is needed to linearly reduce the output amplitude and at the same time avoid introducing harmonic distortion.
Heretofore, so-called "hard-of-hearing" amplifier devices have been utilized as a solution to weak incoming voice signals and high background noise environment problems. Illustrative of these devices if the Western Electric Co. 153B amplifier. However, such devices do not provide for compression of large amplitude incoming signals, with the result being that the listener is left vulnerable to very high sound pressure levels at the ear termed "acoustic shock."
Another device heretofore utilized to provide signal gain, yet compress received audio signals to a specific maximum level without harmonic distortion, is the Model ROC 0300 manufactured by Plantronics, Inc., Santa Cruz, Calif. 95060.
Yet another device for receiver condition is the SOUNDGUARD.RTM. receiver conditioning circuit offered by Plantronics, Inc. This device is the subject of U.S. Pat. No. 4,536,888 issued Aug. 20, 1985, for "Voice Communication Instrument System With Line-Powered Receiver Conditioning Circuit" and assigned to Plantronics, Inc.
The prior devices which compress incoming signals do so regardless of received signal frequency. Compressing peak signals to less than 90 dB across the frequency spectrum will cause some loss of intelligibility. For example, a substantial difference in apparent receive loudness level exists between zero loop calls, such as calls within a building, and calls made through a central office. This is because of the rising frequency response on a zero loop call, whereas it is flattened out or even reduced on long loop calls.
The present invention seeks to provide an improved communications link interface which affords enhanced voice signal conditioning, particularly for received signals, in recognition of human speech/hearing characteristics. Human voice amplitude in normal speech is greatest between the frequencies of 300 Hz and 600 Hz. After attaining a peak amplitude in the vicinity of 600 Hz, voice amplitude tends to fall off rather rapidly. At a frequency of 3.3 KHz, voice amplitude may be down approximately 15-20 dB.
Human hearing sensitivity is low in the lower frequency ranges and increases as the frequency approaches the human voice frequency band. That is, human hearing sensitivity increases to a relatively higher level near 300 Hz. Between 300 Hz and 1000 Hz, hearing sensitivity is nearly constant. However, at approximately 1 KHz, hearing sensitivity begins to significantly increase. Hearing sensitivity peaks at approximately 3 KHz and then rapidly falls off. Increased hearing sensitivity in the 1 KHz to 3 KHz range is beneficial in terms of recognizing and understanding human speech in that frequency range. However, the significantly greater hearing sensitivity in this frequency range makes a person susceptible to acoustic shock and/or annoyance from high amplitude acoustic signals in this frequency range.